The use of transparent glazing materials utilizing polycarbonate resin as a structural component for windows, windshields, and the like are well known. While these polycarbonate resins are easily fabricated into the desired shape and have excellent physical and chemical properties, such as being less dense than glass and having more breakage resistance than glass, their abrasion and chemical solvent resistance is relatively low.
In order to overcome this relatively low abrasion resistance and to otherwise improve the surface characteristics of the polycarbonate, various coatings have been applied onto the polycarbonate substrate. U.S. Pat. No. 3,582,398 describes a fabricated polycarbonate part having improved optical properties consisting of a polycarbonate substrate having a transparent coating thereon consisting of a thermoplastic polymethyl methacrylate. U.S. Pat. No. 4,061,652 describes a coating for polycarbonate resins comprised of (i) an acrylic resin which is a mixture of olefinically unsaturated organic monomers in combination with an acrylic polymer, and (ii) certain urethanes of hydroxybenzotriazoles and hydroxybenzophenones in combination with certain catalysts. U.S. Pat. Nos. 3,451,838, 3,986,997 and 4,027,073 disclose organopolysiloxane coating compositions and techniques for the application of these organopolysiloxane coatings onto polycarbonate surfaces. While these coatings have many desirable properties, e.g., they are hard, abrasion resistant, and chemical solvent resistant, these organopolysiloxane coatings do not in all instances possess the requisite degree of uniform adherence to and durability on these polycarbonate surfaces. U.S. Pat. No. 3,707,397 describes a process for providing a hard coating on, inter alia, polycarbonate articles, said process including priming the polycarbonate surface with an adhesion promoting thermosettable acrylic and applying an organopolysiloxane onto the primed surface. This reference further teaches that the thickness of the thermosettable acrylic polymer primer layer varies between 0.01 mil up to as much as 0.5 mil or even more.
However, organopolysiloxane coated polycarbonate articles which contain a thermoset acrylic primer have suffered from the problem of loss of adhesion of the organopolysiloxane top coat to the polycarbonate substrate upon exposure to weathering.
It has now been discovered that if the primer layer is loaded with high amounts of ultraviolet light screeners, i.e., contains from about 15 to about 75 weight percent of at least one ultraviolet light screening or absorbing compound, then the adhesion of the organopolysiloxane top coat is not deleteriously affected upon exposure to weathering.